RHÖNITE
Rhönite – Ca4[Mg8Fe3+2Ti2]O4[Si6Al6O36] – is a fairly rare inosilicate, characteristic of alkaline rocks. It is not an ore.
It is classified in the Sapphirine Supergroup, Rhönite Group, along with aenigmatite.
It can form prismatic to tabular crystals several centimeters long. It also occurs in skeletal grains with well-developed crystal faces and as anhedral grains.
1. Characteristics
Crystal system: Triclinic pinacoidal, pseudo-monoclinic.
Color: Reddish-brown, black to reddish-black.
Habit: Prismatic to tabular, skeletal, granular anhedral, and in aggregates.
Cleavage: {010} good, {001} good.
Tenacity: Brittle.
Twinning: Common, polysynthetic.
Fracture: No information available.
Mohs Hardness: 5 – 6
Parting: No information available.
Streak: Reddish brown.
Lustre: Vitreous, submetallic.
Diaphaneity: Transparent.
Density (g/cm³): 3.4 – 3.76
2. Geology and Deposits
Rhönite occurs as a primary mineral in silica-undersaturated, alkaline, mafic to intermediate igneous rocks. In the type locality (Rhön Mountains, Germany) it occurs in a nepheline-basanite. It can form as an alteration product of amphiboles.
It also occurs at the contact (skarn) between alkaline basalts and limestones.
It has been found in several meteorites.
3. Mineral Associations
It is associated with pyroxenes (augite-Ti, diopside), amphiboles (kaersutite), alkali feldspars, olivine (forsterite), spinel, perovskite, oxides (hematite, magnetite-Ti), magnesioferrite and erionite.
4. Transmitted Light Microscopy
Refraction indices: nα: 1,795 – 1,810 nβ: 1,806 – 1,825 nγ: 1,830 – 1,845
PLANE POLARIZED LIGHT – PPL
Color / Pleochroism: Dark colors and strong pleochroism:
x = dark reddish-brown, reddish-brown.
y = dark reddish-brown to greenish.
z = dark reddish-brown to black.
Relief: Very high.
Cleavage: {010} and {001} good.
Habits: Prismatic to tabular, skeletal, granular anhedral and in aggregates.
CROSSED POLARIZED LIGHT – XPL
Birefringence and Interference Colors: Maximum birefringence of 0.035, quite high, corresponding to intense and diverse colors up to the end of the 2nd order.
Extinction: Probably oblique.
Elongation sign: No information available.
Twins: Polysynthetic twins parallel to {010} are common.
Zoning: No information available, but probably not.
CONVERGENT LIGHT
Character: B(+)
2V angle: 50 – 90º
Alterations: Through metamorphism or hydrothermal processes rhönite alters to chlorite, serpentine, amphibole, biotite, magnetite, hematite and calcite.
May be confused with: other minerals with strong pleochroism.
Brown hornblende (kaersutite) occurs in the same paragenesis, but the cleavage is different.
Spinel is also colored, but it does not exhibit pleochroism or only weak pleochroism and is isotropic.
Aenigmatite is very similar, but the birefringence is much lower and it occurs in another paragenesis.
5. Reflected Light Microscopy
Reflected light microscopy is clearly not the recommended analytical method for identifying rhönite. However, it is important to prepare a polished slide or section to identify opaque minerals that occur associated with rhönite, such as magnetite and hematite.
There is no data on rhönite under reflected light, and samples are not available due to the mineral’s great rarity.